1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition, and more particularly, to a piezoelectric ceramic composition having improved heat resistance and piezoelectric property by replacing conventional piezoelectric ceramic components with other components, and a piezoelectric ceramic device using the same.
2. Description of the Related Art
As a result of recent developments in the information industry, almost all appliances and devices such as Hard Disc Drive (HDD), Hand-Held Phone (HHP), Compact Disc Read Only Memory (CD-ROM), CD-Recordable and Writeable (CD-RW), Digital Versatile Disc (DVD), DVD-Recordable and Writeable (DVD-RW), wireless keyboard, optical mouse or the like are provided with a SMD-type resonator producing a clock frequency, as essential parts in the digital era. Keeping pace with the performance enhancement and miniaturization trends of digital products, the resonator continues to advance toward higher frequencies and a small, light and power-efficiency. In order to manufacture more and more miniaturized resonators using a higher frequency, a piezoelectric ceramic composition having excellent piezoelectric property and thermal stability is required.
A Pb(Zr,Ti)O3- and a PbTiO3-based piezoelectric ceramic composition have been mainly used in recent years. A thickness vibration mode is a representative vibration mode in these piezoelectric ceramic compositions. The thickness vibration mode uses energy trap vibration and is divided into a fundamental vibration mode and a higher order vibration mode.
As for the Pb(Zr,Ti)O3-based composition, contrary to a fundamental vibration mode, piezoelectric property is relatively poor in a higher order vibration mode. For the higher order vibration mode, furthermore, because a resonance frequency constant is relatively small, there is a limitation in that it is difficult to obtain a high frequency. On the other hand, the PbTiO3-based composition has such characteristics as a low dielectric constant, a small grain size and a high phase transition temperature. Furthermore, due to large lattice anisotropy in the C-axis direction, it is difficult to trap energy in a fundamental vibration mode, but it is easy to trap energy in a higher order vibration mode capable of producing a high frequency. In light of these facts, the PbTiO3-based composition has superior piezoelectric property and thermal stability to the Pb(Zr,Ti)O3-based composition. Accordingly, the former has been recognized as a material suitable for a high frequency band oscillator.
Nevertheless, because the PbTiO3-based composition has large crystal anisotropy, there is a problem in that spontaneous phase transition stress during cooling readily causes cracks, resulting in a sintering property being poor. Recent studies have been proceeding actively with the aim of minimizing spontaneous phase transition stress during cooling and enhancing a sintering property by the substitution of A-site and/or B-site by other components or the use of other additives.
By way of a representative example, Japanese Patent Laid-Open Publication No. 7-206517 (publication date: Aug. 8, 1995, applicant: MATSUSHITA ELECTRIC IND CO LTD) discloses a piezoelectric ceramic composition comprising a (Pb,La)TiO3-based main component and CuO, ZrO2 and MnO2 as additives. The piezoelectric ceramic composition presented in the above publication has a high usable frequency and a large D/R (dynamic ratio) during resonating. In spite of these advantages, the piezoelectric ceramic composition must be sintered in an atmosphere having an oxygen partial pressure of above 80%. The composition is used as a material of a piezoelectric ceramic device with a diameter of 15-18 mm; however, the composition cannot be used as a material of a latest piezoelectric ceramic device for driving a hard disc, which has an electrode area of 2.5 mmxc3x972.0 mm and produces a frequency of 40 MHz or more. When the device is miniaturized, there are problems in that a D/R is reduced during resonating and a spurious phenomenon occurs in third harmonic frequency.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a piezoelectric ceramic composition, which is used as a material of a piezoelectric ceramic device with an electrode area as small as 2.5 mmxc3x972.0 mm or less.
It is another object of the present invention to provide a piezoelectric ceramic composition, which can be sintered under normal atmosphere.
It is a further object of the present invention to provide a piezoelectric ceramic composition, which has excellent electric property (D/R=above 60) and thermal property under reflow at about 250xc2x0 C. or more.
It is another object of the present invention to provide a piezoelectric ceramic composition, which exhibits excellent piezoelectric property at a thickness longitudinal vibration third harmonic frequency.
It is yet another object of the present invention to provide a miniaturized piezoelectric ceramic device, which produces a high frequency and is able to be mounted in a surface mount manner at a high temperature.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a piezoelectric ceramic composition represented by the general formula (1) below, which comprises essentially Pb, La, Ti, Mn and Cu:
{[Pb(1xe2x88x921.5x)xc2x1(0xcx9c0.2)Lax][Ti(1xe2x88x92yxe2x88x92z)MnyCuz]O3}xe2x80x83xe2x80x83Formula (1)
(wherein, 0.02 less than x less than 0.14, 0.01xe2x89xa6y less than 0.04, and 0.002xe2x89xa6zxe2x89xa60.008.)
In accordance with another aspect of the present invention, there is provided a miniaturized piezoelectric ceramic device, which produces a high frequency and is able to be mounted in a surface mount manner at a high temperature.
A piezoelectric composition of the present invention is expressed by the general formula, [Pb(1xe2x88x921.5x)xc2x1(0xcx9c0.2)Lax][Ti(1xe2x88x92yxe2x88x92z)MnyCuz]O3. The composition is an ABO3 composite perovskite composition, the A-site consisting of PbO and La2O3, and the B-site consisting of TiO2, MnO2 and CuO. Contrary to a conventional composition, manganese (Mn) and copper (Cu) are not additives, but are substituted for titanium (Ti) as a matrix composition. As a result, charge neutrality of grains and grain boundaries, along with crystal structure and chemical stability, may be more stably adjusted.
In the above formula, lanthanum (La), an A-site component, is required to meet 0.02 less than x less than 0.14. If the x value is more than 0.014, cavities are formed in some sites occupied by Pb and thus charge neutrality is maintained. As a result, diffusion of atoms occurs to reduce internal stress due to crystal anisotropy during sintering and to prevent the formation of cracks. However, if the x value is below 0.02, such effects are not sufficiently realized, and if it exceeds 0.14, oscillation stability and thermal stability are poor.
In the above formula, Mn and Cu, B-site components, present in a small amount, serve to enhance electrical properties and thermal stability of a piezoelectric ceramic composition. Mn is required to meet 0.01xe2x89xa6y less than 0.04. If the y value is more than 0.04, because Mn acts as an electron acceptor, there are problems in that the insulation property of a sintered body is low and an amount of leakage current increases rapidly, thereby preventing polarization or lowering piezoelectric property. Cu is required to meet 0.002xe2x89xa6zxe2x89xa60.008. Within the above range, cavities are provided, so as to lower resonance resistance and thus control an amount of leakage current. If the z value is outside the range, piezoelectric property is low and thermal stability is poor.
The piezoelectric ceramic composition of the present invention is excellent in sintering property and thus is able to be stably sintered even under normal atmosphere. Accordingly, there is no need for the composition to be sintered under an atmosphere with controlled oxygen partial pressure. Furthermore, it has excellent electric property under reflow above 250xc2x0 C. and has excellent thermal stability even after reflow. Accordingly, it exhibits excellent piezoelectric property at a thickness longitudinal vibration third harmonic frequency capable of producing a high frequency. Also, the use of the composition makes it possible to manufacture a piezoelectric ceramic device having an electrode area of 2.5 mmxc3x972.0 mm or less.
To accomplish an excellent reflow property, reflow must be carried out at about 240-250xc2x0 C., and to maintain polarity and a large D/R value, a Tc value must be above 295xc2x0 C. Also, if a value of D/R is lower than 60, an amount of battery wastage, i.e., an energy loss is large. Accordingly, it is preferred that a Tc value is not only above 295xc2x0 C. but also a D/R value is above 60.